lecture 4 crystal growth
TRANSCRIPT
8/8/2019 Lecture 4 Crystal Growth
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Silicon Wafer
Manufacture
Packaging
Epitaxial
GrowthOxidation
Photo-
lithography
Etching
Diffusion (IonImplantation)
Metallization
Fabrication Processes for VLSI Devices
Chip Fabrication Processes
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Silicon Wafer Preparation
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Silicon Wafer Preparation
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Czochralski (CZ) Growth Method
• CZ is more commonmethod to grow
silicon crystal todaybecause it is capableof producing largediameter crystals,from which largediameter wafer canbe cut.
Lecture # 4
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Modern CZ Crystal Growth
• The raw Si used for crystalgrowth is purified from SiO2
(sand) through refining, fractionaldistillation and CVD.
• The raw material contains < 1 ppbimpurities except for O (» 1018
cm-3) and C (» 1016 cm-3)
• Essentially all Si wafers used forICs today come from Czochralskigrown crystals. Polysilicon materialis melted, held at close to 1415°C, and a single crystal seed isused to start the crystal growth.
• Pull rate, melt temperature androtation rate are all importantcontrol parameters.
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CZ crystal growth (cont.)
• Sequence of
photographs anddrawings illustratingCZ crystal growth.The charge ismelted,
• the seed isinserted, the neckregion is grown at ahigh rate to removedislocations andfinally the growth is
• slowed down to
produce a uniformcrystal.
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300-mm (12 in.) and 400 mm (16 in.) Czochralski-grownsilicon ingots. (Photo courtesy of Sin-Etsu Handotai Co.,
Tokyo.)8
Wafer Slicing
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Drawback of the CZ method
• The only significant drawbackto the CZ method is that thesilicon is contained in liquidform in a crucible duringgrowth and as a result,impurities from the crucibleare incorporated.
• in the growing crystal. Oxygenand carbon are the two mostsignificant contaminants.
• These impurities are notalways a drawback, however.
Oxygen in particular can bevery useful in mechanicallystrengthening the siliconcrystal and in providing ameans for gettering otherunwanted impurities duringdevice fabrication.
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Modeling Crystal Growth: relationship between pull
rate and crystal diameter.
• Freezing occurs between
isotherms X1 and X2.
• Heat balance: latent heat of
crystallization + heat conducted
from melt to crystal = heat
conducted away.
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A= Cross-sectional area
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2πrdx=radiation surface
area of an incremental
Length. σ=Boltzman constant
KS=thermal conductvity of the soild
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V pmax = maximum crystal pull rate is inversely proportional to the square root of the
crystal radius. 14
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Modeling Dopant Behavior During CZ Crystal
Growth
• •Dopants are addedto the melt to
provide a controlled• n or p doping level
in the wafers.
• However, the dopantincorporationprocess iscomplicated by
dopant segregation.
Ko is the segregation coefficient.CS
and CL are the impurity concentration
just on the either side of the solid/
liquid interface.
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VO=initial volume
IO=number of impurities
CL=impurity concentration
n the melt.
Lecture # 4
All values are below 1,which means that duringGrowth the dopants areRejected into the melt.
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17f= Vs/Vo, a fraction of melt that has solidified 18
Doping concentration versus position along the grown CZ
crystal for common dopants in silicon.
Consider three cases: If K<1If K>1if K~1
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Curves for growth from the melt showing the doping
concentration in a solid as a function of the fraction
solidified.20
Float Zone Growth Technique
• CZ wafers are contaminated by O2 and Cfrom the crucible or graphite heaters.
• This limits the resistivity to ~ 20 Ωcm,while intrinsic Si is 230 kΩcm.
• Extremely high purity Si wafers aremade using float zone growth.
• FZ does not use a crucible or carbonheaters.
• * More expensive.
• * Carrier concentrations down to 10 11
atoms/cm 3 have been achieved.• * High purity needed for power
thyristors and rectifiers.
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Float-zone process. (a) Schematic setup. (b) Simple model for
doping evaluation. 22
Float Zone Growth Technique
• A seed crystal is brought into contact withthe top of the rod.
• In an inert atmosphere, an RF coil is slowlypassed along the length of the rod, startingat seed contact.
• The field set up by the coil induces eddycurrents in the rod, leading to jouleheating, and so melts the rod in the vicinityof the coil.
• The "floating" melt zone is about 2 cmwide..
• The seed crystal touches the melt zone andis pulled away, along with a solidifying Siboule following the seed.
• The seed crystal determines the crystalorientation of the boule.
• Limited to about a 4" wafer, as the meltzone will collapse.
• It is only held together by surface tension(and RF levitation).
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FZ Growth Considerations
• Segregation and evaporation of impurities in the melt zone helppurify the Si further.
• * Recall, if k < 1, then more dopants/impurities in the liquid thanin the solid.
• * Thus, the impurities generally stay in the melt zone, and don'tsolidify in the boule.
• * You can "purify" FZ wafers further by successively passing thecoil along the boule. The impurities then segregate towards theend of the boule.
• Thermal instability in the melt zone can cause microvariations incomposition and doping.
• Difficult in making a uniform dopant concentration.24
Doping in FZ Growth
• Gas Doping:• Dopants are introduced in gaseous form during FZ growth.• * PH 3 (Phosphine), AsCl3--> n-dopant
• * B 2H6 (Diborane), BCl3 -> p-dopant• * Good uniformity along the length of the boule.
• Pill Doping:• Drill a small hole in the top of the EGS rod, and insert the dopant.
• * If the dopant has a small segregation coefficient, most of it will becarried with the melt as it passes the length of the boule.
• * Resulting in only a small non-uniformity.
• * Ga and In doping work well this way.